On the night of 14 Rabīʿ al-Awwal 170, the caliph al-Hādī died in Baghdad and the oath passed to his younger brother Hārūn, styled al-Rashīd — by the chronicles' fondness for coincidence, the same night his son al-Maʾmūn was born.
Hārūn's twenty-three-year reign sat at the material peak of the ʿAbbāsid caliphate: the Baghdad of the paper mills and translation movement's beginnings, of Barmakid administration and pilgrimages led in person, of embassies exchanged with Charlemagne. It was remembered — and embroidered — until "the age of Hārūn al-Rashīd" became shorthand, in the Thousand and One Nights and far beyond, for the golden age of the city itself.
Al-Ṭabarī records the accession date without hesitation, one of the cleanly fixed political dates of the era.